Robots usually aren't supposed to hurt people. But how does a robot know if its actions injure a human? The only way is for the robot to learn how much physical pressure the human body can comfortably endure. So Borut Povše, a researcher at the University of Ljubljana in Slovenia had a robot punch people to find out:

Each volunteer was struck 18 times at different impact energies, with the robot arm fitted with one of two tools - one blunt and round, and one sharper.

The volunteers were then asked to judge, for each tool type, whether the collision was painless, or engendered mild, moderate, horrible or unbearable pain. Povše, who tried the system before his volunteers, says most judged the pain was in the mild to moderate range.